What if?
Disclaimer: I am an organic chemistry graduate student.
What I'm sharing with you are observations and suggestions based on a few hours of reading on the topic, a basic understanding of the chemistry of cannabis, and several years lab experience in purification processes.
I am by no means a Dr. of chemistry and do not purport myself to be, however--I have gained a reasonable amount of knowledge on the subject and may have some insight to add. What I can say is that I know what I'm suggesting won't destroy the target chemicals. The method may require some tweaking, but ultimately none of these concepts should be super unfamiliar to you.
That said:
For any of you guys doing this. I'd be interested to see what the results of the following procedure would be.
1. Extract as usual, with an added caveat that you should keep the butane/oil mixture hot as it is being eluted into your container ("blast" into a container over a boiling water bath or pre heated heating element--No flames or sparks obviously).
Notes:
Your best chance to prevent what are called "inclusion" or (if they are larger sized) "occlusion" impurities when "working up" (removing solvents/purifying/isolating product) a reaction involving production of a viscous oil is to control the viscosity the oil can attain as the impurities work their way out. For a lot of mathy reasons this is the best place to increase the efficiency of your removal of butane.
This step alone (with some optimization) might increase the efficiency with which you remove butane significantly. I can't say what the best temp will be without knowing specifically the viscosity of BHO and other factors.
Here is where my proposed experiment deviates from the standard (or at least what I've read in the past few hours):
2. Dissolve the the BHO in the highest grade ethanol (EtOH) that you can find. The mixture should be homogenous, and have low, water-like, viscosity.
Notes:
I choose ethanol because it dissolves the target chemicals and is much safer for consumption than butane in general. If you are concerned that butane is bad for your health and should be purged, you may be best suited to all but remove it completely by driving it out with an excess of another solvent. Furthermore, butane has a higher affinity (through intermolecular interactions) for the target molecules according to information I can find than does ethanol. This makes it more likely, on evaporation, to "include" itself as micro-droplets in oil. You might say, well why not just use ethanol for the first extraction? The answer is because it tends to extract more unwanted molecules from the plant which butane excludes. In effect the extraction tube you guys are using is a column chromatograph, the added dimension of slightly higher pressure created by the butane rapidly expanding is actually beneficial in this. Read up on how polarity affects column chromatography and you will understand better why butane is getting out more of what we want and less of what we don't. In any case butane's high affinity for the target is beneficial initially. Now that we have reached this step of the purification process its utility is greatly diminished due to this same property, as it will tend more to include itself in the oil.
3. Heat the resulting liquid with vigorous stirring and a fan blowing into the receptacle for ventilation. I must strongly recommend you use all due precaution for ventilation and avoidance of exposure to flame during this step. A good minute or two of this with the liquid brought to sufficient heat (hot to the touch is more than good enough) should be enough to drive off any excess butane.
4. evaporate the alcohol using any number of methodologies easily looked up and collect oil.
Notes:
You may vacuum purge this if you like, but you will need to reconsider optimal pressures and temperatures.
Several cycles of heating to a molten state and cooling will help to drive off excess EtOH in absence of a purge. Stirring during this process will help tremendously.
In closing:
The reason I suggest this process is that, for this application and with readily available equipment, it will be impossible to remove all butane by any other method. Across the web I see much talk of which method will really remove ALL the butane. The answer is, sans a lab, training, and a painful sacrifice to yield--none of them.
Realistically the solvent system available for this extraction is terrible--no chemist would extract these chemicals this way. However, without proper training it is dangerous and borderline unethical to suggest using anything else to perform it. Some of the stuff I've seen on the web, fuming sulfuric acid for one, can be fatal if mishandled.
Truthfully, if I had my way--everyone would be using hexanes to extract and then performing water washes to remove impurities which are likely to contribute to the inclusion of solvents in products. Subsequently hexanes could be driven off using the above described method. I believe this would greatly increase purity. Hexanes has an even higher affinity for THC than does butane which helps with initial extraction as well.
Ethanol is not as toxic as butane--and though its not awesome to smoke still, its significantly better than imploding a mason jar in your kitchen to the sound of your screaming wife.
Smoking even trace amounts of butane over and over and over again may do tangible damage over time. According to what I can find the same is true for ethanol, but it is less true. All of this must be taken in context, though, you could inhale butane as a hobby and never have complications. Or inhaling it once in your life might give you cancer, chemistry is funny like that.
Let me know if you have any questions on what I'm getting at, or how to set up a control experiment to test results.